The present invention relates to the art of electrographic recording systems and more particularly to an apparatus and method for forming electrostatic latent images on a recording medium with information provided by electronic signals.
This invention is particularly applicable to the color electrographic art and apparatus suitable for consecutively forming latent component images which are subsequently developed with a respective color developer and which are superimposed to produce a composite color image on the recording medium.
The electrographic recording process for producing a visible image can be generally characterized as two step process. The first step in an electrostatic recording image is formed on a record medium at a recording station having a recording head. One version of the recording head comprises a plurality of recording or writing electrodes or multistylus physically positioned to electrically address a dielectric surface of the recording medium as the medium travels through the recording station. Oppositely opposed to these writing electrodes is an aligned series of backup electrodes. These oppositely opposed writing and backup electrodes form a printing gap. An electrostatic charge is deposited on the dielectric portion of the recording medium as the medium is moved through the gap when the potential difference between addressed writing electrodes and the oppositely opposed backup electrodes is raised to a threshold level, also referred to as the Paschen breakdown point, comprising several hundreds of volts. The timing sequencing of energization of the electrodes provides for electrical charging of selected areas of the medium to form a desired latent image as the medium is moved through the recording station.
The second step is rendering the electrostatic latent image visible by toning or developing the latent electrostatic image deposited on the medium. Development may comprise liquid or dry toner. In the latter case, some type of fusing of the toner to the medium may be employed, e.g., flash fusing. In the former case, provision may be made to aid in the removal of excess toner followed by the drying of the liquid toned medium surface. As a result, a permanent fixed printed record is formed on the recording medium.
Commercially successful electrographic apparatus in the past has involved one recording station and one development station with one toning medium, which is usually black. In recent years there has been increasing pressure on electrographic system vendors to create and develop reliable color electrographic systems. This is particularly true because of the increased employment of color CAD system usage and color LSI/VLSI design mandating the use of color display and design and, consequently, requiring color hard copy output.
While many color hardcopy systems have been proposed and designed, most have not been as successful in meeting the requirements for good color output except possibly for the color pen plotter systems. Such systems are provided with multiple color pens that trace according to electrical signal input to produce a multicolor output on a recording medium. However, such systems also have drawbacks in that full, filled in and uniform color areas or zones on the medium are not readily possible as is in the case of electrographic systems using a color or black developing medium. This would be desirable in applications such as VLSI chip layouts. Going from single color to multiple color electrographic recording, a complex problem is encountered. In single color recording, the latent image is formed followed by its immediate development. However, in color recording, two or more latent images must be independently formed and developed and superimposed upon one another. Thus, some means of registration is mandatory to insure that the different color component images are sufficiently superimposed to prevent color fringes and color errors and provide a high resolution color print.
Series color electrographic systems are not new. An example in the xerographic arts is disclosed in U.S. Pat. No. 4,286,031. In this patent, a plurality of latent electrostatic images of different colors in registration are achieved to produce a composite color image. It is not specifically mentioned as to how superposition of the component images is accomplished.
The problem of color image component registration and a proposed solution was recognized in U.S. Pat. No. 4,007,489. In this patent, a series of recording/developing stations are provided, i.e., each station is provided with a recording head and respective color developer. Component image registration is accomplished by the use of a printed cross mark placed on the medium surface which is sensed by a series of sensors before each recording/developing station. However, the use of a single registration mark and multiple recording stations has several drawbacks. displacement both in the lateral and longitudinal directions of the record medium can easily occur because of the differences (1) as to time placement of the latent images being formed, and (2) as to the physical parameters of the different recording head structures. Also, there are variations in the position of the recording medium as it moves through the system from station to station. Also, the recording medium will change in size, i.e., it will stretch or expand, or shrink or contract both laterally and longitudinally relative to its length. Such expansion or shrinkage is sufficient to produce low composite image resolution due to color fringes and color registration errors and result in component image misalignment.
It is not precisely known as to all the factors that might contribute to dimensional changes in the recording medium but it is believed that the major factors involved are medium handling, image development and image fixing. In the usual case of recording medium comprising dielectric coated paper, the medium can stretch or shrink as much as 1 mil per foot and the dimensional change laterally across this type of medium can be three times greater than the dimensional change along the longitudinal extent of the medium. The medium is acceptable to such dimensional changes due to the manner by which it is made. For example, the fiberous grain of the paper base of such recording media is such that it can stretch or shrink more in one orthogonal direction as compared to another. Other recording media, e.g., polyester based transparent recording media, may not stretch or shrink as much as paper based media, but are still succeptable to stretching and shrinkage.
Further, paper based recording medium is neither precisely flat or straight nor are the medium edges exactly parallel to one another.
These medium dimensional changes and physical irregularities which occur while the medium is moving through the color electrographic system will contribute significantly to providing color component image registration that is not sufficiently acceptable for commercial usage without adequate color component image registration.
While one solution to this problem might be to require tighter specifications in the design and manufacture of recording media without these irregularities, this would not be desirable because of the high costs to provide such quality control in the manufacture of this media, which would not be acceptable to media manufacturers. The better approach is to create a color electrographic machine that is capable of "living" with these irregularities and capable of controlling the registration of sequential composite latent images by measurement and control of machine functions without requiring changes to the recording medium.
As previously indicated, the proposed solution of U.S. Pat. No. 4,007,489 is to provide a mark on the recording medium. The mark in the form of a cross is provided as a latent image at the first recording station outside the image field where it is also developed as a visible image. The mark is then photoelectrically sensed at subsequent recording stations. The sensor comprises a set of laterally disposed photodiodes. Sensing in the lateral or y direction of the recording medium, i.e., parallel to the aligned electrodes, is accomplished by determining a deviation in the position of the horizontal line bar of the cross mark and thereafter controlling which electrode in the aligned electrodes at the respective station will serve as a start point for a line of recorded information. Sensing in the longitudinal or x direction of the recording medium is accomplished by determining the beginning of the vertical line bar of the cross mark.
The drawback of this registration system is principally two fold. First, a different set of recording electrodes is present at each station having their individual character as to variations in electrode spacing and alignment and electrode head warpage. Secondly, no provision is made for continuously monitoring changes in medium expansion or shrinkage and take corrective action based upon those changes. The single cross mark is placed on the medium from which lateral and longitudinal approximation is accomplished for superimposition of the component latent images. Nothing is taken into account for handling on-the-fly correction relative to the disposition of latent component images based upon measurements made electronically that are indicative of recording medium irregularities.